Joining a 1972-2006 AMC I6 V8 engines to GM SM420 is a viable and worthwhile option. Their power will be complimented by the strength and gearing of the Muncie 420. The parts and procedure are detailed below.
Two kits along with multiple options below are available to meet your needs.
This option is best for those who have their own bellhousing. The retainer kit and machining operations are listed as separate options.
Special SM420 retainer machined from a billet steel to Late (seal style) SM420 dimensions on the transmission side and AMC bellhousing bore and sleeve dimensions on the engine side. Four 5/16-18 socket head (Allen) bolts, input seal and application specific pilot bushing are included.
Some SM420 transmissions have a shift rail protrusion that will interfere with AMC bellhousings. This pocket is machined to provide clearance along with the correct pattern to bolt up the transmission.
This option adds a machined bellhousing to the Transmission Bearing Retainer Kit above for those needing a bellhousing. If you also need a CPS, add the appropriate option on below.
The kit additionally includes a machined bellhousing, clutch release arm, boot, release arm pivot, return springs, pilot bushing bellhousing lower shield plate, as well as the Transmission Bearing Retainer Kit listed above.
1972-2006 AMC I6 & V8 engines, 232, 258, 4.0L, 304, 360, and 401 engines are all valid candidates.
Not all AMC crank flange bores are the same. Early engines, 1971 and previous - as well as those with automatic transmissions around this era - featured a ~1.8" step (see image to the right) or a shallow recess instead of a ~1.8" centering bore as found on later AMC V8 engines. This distinction is important to make if adapting to a transmission where the pilot bushing is positioned further rearward due to a shorter input shaft length. All early and late cranks (in our documentation) feature a pilot bushing bore (the deeper bore in the crank) that is just over 1" in diameter whether they have the above step-bore, or not. On the later engines you are placing the pilot bushing where an automatic transmission would normally index the torque converter. Also be aware that on AMC engines originally equipped with a TH400 automatic transmission there may be a thin centering ring that will need to be removed to install the larger pilot bushing. If the bore measure ~1.7” instead of the expected ~1.8” you’ve got the bushing that needs to be removed. That ring 5352374 is located here.
The factory flywheels on all of the above engines are acceptable. Most will accept the recommended 10-1/2” clutch. There are instances of larger clutches in some trucks. If you have one of these applications the clutch diameter will need to match.
Novak’s hydraulic release slave cylinder kit.
The factory pressure plates, whether three-finger or diaphragm, are compatible with this adaptation. We have found that the diaphragm style pressure plate generally works better with hydraulic applications. 1976-1979 (T150 & T18) clutch disc hubs have a 1-1/16" spline diameter that is not compatible with the 1-1/8" input shafts of the SM420.
The factory clutch release arms and their linkages can be retained including mechanical bellcrank style linkages. However, now is the perfect time to convert to an hydraulic clutch release system which has many advantages. Components are listed below.
While dated, the SM420 offers an excellent low ratio with overall compact size and weight. Parts are getting hard to find these days, so you may want to stockpile any spares you can locate. All versions of the SM420 that use a standard 1-1/8” input shaft are compatible with this conversion. Because the front retainer includes a seal, early transmissions not equipped with a seal originally will require the input shaft machining option below that requires your good shaft be sent in. We do offer fully remanufactured SM420 transmissions with all our available updates and can configure it with these parts as a package. If you want a solid SM420, this is a great option and is listed below.
The bellhousing will need to be machined to accept the SM420 bolt pattern and bore of the retainer kit. This option is for that service on your CLEAN bellhousing or a Novak provided bellhousing.
***Please note - our CNC operators are very protective of our valuable CNC mills and won’t allow anything dirty in them. A charge of $50 will be imposed on any bellhousing that is not clean enough to enter their mill.***
This photo illustrates a typical “early” 10 o’clock CPS installation.
This includes machining a clean customer-supplied or Novak supplied I6 / V8 bellhousing to the CPS pattern selected including necessary hardware for the sensor. Sensor is not included but is available below.
***Please note - our CNC operators are very protective of our valuable CNC mills and won’t allow anything dirty in them. A charge of $50 will be charged for any bellhousing that is not clean enough to enter their mill.***
If you are running a stock fuel injected engine, you will need a CPS location on the bellhousing along with the correct flywheel carrying a matching reluctor. The Early position is about 10 o’clock looking from the rear of the engine and 2004-06 is about 3 o’clock.
Does not apply to AMC 2.5L applications.
One of the following Pilot Bushings is required for all installations. Please choose your applicable engine and transmission combination:
This popular clutch assembly from 1980-1986 Jeeps is our preferred kit for holding power yet reasonable pedal pressure. The disc's spline hub features the correct 1-1/8" x 10 spline configuration for these transmissions.
The disc can be purchased separately for those early applications natively using the 10 spline 1-1/16” disc
While often not needed in this application, it is priceless when clearances dictate its use. Further information is here.
While sometimes not needed in this application, it is priceless when clearances dictate its use. With all the variabilities in flywheel and clutch height, do recommend its use. Further information is here.
Though not required on all applications, now is an excellent opportunity to upgrade from the likely worn mechanical system. These mechanical systems are usually full of play which is aggravated by years of wear and much more affected by flex in the body/frame relationship, especially when in a twist off-road. You will find that your clutch release point will vary as things twist. With an hydraulic system, that is not the case. All flex is taken up by the braided stainless line which is unaffected by changes in frame to body relationship. If your Jeep has the master cylinder, that part can be reused. More information can be found here.
Parts to do this swap are:
Some would rather not have to hassle with a questionable transmission and trying to source all the necessary pieces to make a good one. We offer expertly re-manufactured and updated complete SM420 transmissions. These will have all the available upgrades including our billet PTO covers, synchros bearings, sealing upgrades, etc.
If you have the early style transmission that did not use a seal on the input, it will need to be machined to accept the lip seal that is in the retainer. Please send in your CLEAN input for this service.
Though not necessarily required, it is our recommendation to do a full rebuild on these units while you are in the transmission. These are the best parts available. Additional hard parts are no longer manufactured and get increasingly more difficult to obtain. If you have pieces that need to be replaced, it is often best to look at the full transmission option above or keep additional cores on hand. If you need other parts, you are welcome to call in. Please don’t be offended if our sales tech chuckles when you ask for first or second gear or simply quotes you the price of a complete unit. Our instructions accompany all rebuild kits. At the very least you will want the gaskets, but we do recommend the full rebuild kit.
Straight sticks are available if yours is work or you simply want to start with a clean slate to get the bends right.
Installation requirements will vary a great deal depending on how much of this conversion you choose to do for yourself. If all the options are purchased, typical mechanic's hand tools will generally suffice. It gets more complicated the further you choose go “go it alone”. Machining the bellhousing requires a mill while transmission rebuild necessitates pullers, a press, large snap ring pliers, etc.
The installation procedure is listed below.
Place the engine flange of the bellhousing face down - preferably on smooth wood or the table/bed of a milling machine. Use a transfer punch to transfer the four bolt pattern of the transmission front flange onto the bellhousing rear face. If a transfer punch is not available, you may use a bolt or pin or drill to transfer the four marks onto this rear face, though this is not preferable.
Align the top cover gasket surface of the transmission with the dowel pin holes in the bellhousing to make sure the trans will be level. Transfer the mounting bolt hole pattern of the transmission to the bellhousing. Drill the two upper holes with a letter "U" or 23/64" size drill and tap these two holes to 7/16-14.
Your 420 will have one of two types of lower bolt holes. One offers clearance for a bolt that comes through the transmission flange ear and threads into the bellhousing, as per convention. The other type of 420 is drilled and tapped for the bolt to screw into from inside the bellhousing. Either of these types will work with this procedure.
Through-Drilled Lower Transmission Ear Version
If the SM420 has a lower driver's side hole that is not threaded, proceed as follows:
Threaded Lower Transmission Ears Version
If the 420 has lower holes that are threaded proceed as follows:
While these written instructions may sound complicated, the actual procedure is really quite simple and the end result is worthwhile.
When working with the 4.0L engine, a crank position sensor is required. We can precisely install most existing sensors (from the installer's wiring harness) or a new sensor into an AMC bellhousing.
Some SM420 transmissions have a shift rail protrusion that will interfere with AMC bellhousings. A pocket may have to be machined to provide clearance.